<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>date</title>

  <style>
    div {
      width: 300px;
      height: 40px;
      border: 1px solid pink;
      text-align: center;
      line-height: 40px;
    }
  </style>
</head>

<body>
  <div></div>
  <script>
    const div = document.querySelector('div')
    // 自定义时间函数
    function getMyDate(){ 
      const date = new Date()
      let h = date.getHours()
      let m = date.getMinutes()
      let s = date.getSeconds()
      h = h < 10 ? '0' + h : h
      m = m < 10 ? '0' + m : m
      s = s < 10 ? '0' + s : s
      return `今天是:${date.getFullYear()}年${date.getMonth() + 1}月${date.getDate()}日 ${h}:${m}:${s}`
    }

    // 定时器-调用时间函数
    div.innerHTML = getMyDate()
    setInterval(function(){
      div.innerHTML = getMyDate()
    },1000)
  </script>  
</body>

</html>